U.S. Pat. No. 5,322,664 assigned to the assignee hereof, discloses a method and apparatus for making a clear single-layer non-foam polystyrene film for use as a sleeve label. A blend of crystal polystyrene containing no mineral oil and a block copolymer is extruded through an annular orifice and then stretched over a cooling mandrel. The film is stretched in both the machine direction and cross direction. The film may be formed into sleeves with the machine stretch direction oriented circumferentially, and then shrunk onto beverage containers.
It is well known that polyolefins, such as polyethylene and polypropylene, have better toughness and strength characteristics than polystyrene. However, polystyrene offers attributes superior to polyolefins in terms of stiffness and machinability. Efforts to overcome some of the limitations in properties of polyolefins, polypropylene in particular, have been to produce films or sheets with high levels of orientation in both machine and cross directions (biaxial orientation). Such techniques require substantial equipment investment and high capacity requirements.
Polystyrene is an amorphous material with excellent stiffness, cutting and machining qualities. It can be blended or polymerized with butadiene or other rubber-type modifying materials to increase its toughness and strength characteristics. It can also be extruded with a much lower equipment investment than biaxial orientation equipment for polypropylene. However, even by using undesirable amounts of liquid plasticizers or very expensive copolymers, the load force necessary to make polystyrene stretch exceeds values for polypropylene and values required in some market applications. One example where stretch under a low force is desirable is with label stock for PET carbonated beverage containers. Carbonated beverages are bottled at low temperature, around 50.degree. F. Labels are applied to the containers either before or after filling with the beverage. These labels are wrapped around the containers in the machine direction axis of the sheet, with the ends being overlapped and bonded by a hot melt adhesive. As the beverage in the bottle warms to room temperature, the pressure created by carbonization makes the container expand, exerting a circumferential force on the container label. For example, a two liter bottle with carbonated content at typical carbonation levels can increase in diameter sufficiently to increase the circumferential dimension of the container 0.10 inches or more. The hot melt adhesives used in the beverage labeling industry are formulated for ease of processing, and typically do not possess high sheer strength at room temperature. It is therefore necessary that the label be such as to stretch at a lower force load than the sheer strength of the adhesive in order to accommodate expansion of the container without fracture at the label seam. This force load is typically less than four pounds. Label stock of high impact polystyrene typically require in excess of four pounds to stretch the required 0.10 inches.
Polyolefins and polystyrene, normally incompatible, can be made compatible by use of a compatibility agent, such as a styrene-ethylene/butylene-styrene block copolymer. However, processing problems created by the nature of the blends of amorphous polystyrene and crystalline polypropylene have resisted commercial applications of this technology. With a significant amount of polypropylene (above 40%) combined with polystyrene, the blend has a low melt strength that creates problems in extruding a sheet, particularly one that has a low caliper and is commercially economically viable. The presence of the polypropylene also makes the extruded sheet sensitive to a condition known as scaling (similar to fish scales), which is an appearance blemish visible as a chevron pattern with variable opacity.
It is therefore an object of the present invention to provide a multilayer sheet and method of manufacture adapted for use as a label sleeve on containers that combine the desirable properties of polyolefins in terms of toughness and strength, and the desirable properties of polystyrene in terms of stiffness and machinability. Another object of the present invention is to provide a system and method for coextruding a sheet film of polyolefin and polystyrene composition that are economical to implement, and that provide sleeve labels having the desirable properties described above. A further object of the present invention is to provide a container for carbonated beverages having a sleeve label in accordance with the present invention secured thereto.